1. Field of the Invention
The present invention relates to new copolymers and use of such copolymer as a resin binder component for photoresist compositions, particularly chemically-amplified positive-acting resists.
2. Background
Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
In general, photoresist compositions comprise at least a resin binder component and a photoactive agent. Photoresist compositions are described in Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, ch. 2, 1975 and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, ch. 2 and 4, both incorporated herein by reference for their teaching of photoresist compositions and methods of making and using the same.
While currently available photoresists are suitable for many applications, current resists also can exhibit significant shortcomings, particularly in high performance applications such as formation of highly resolved sub-micron and sub-half micron features.
For example, problems can arise upon etching a bared substrate surface defined by a developed resist relief image. In particular, to etch aluminum, silicon oxide and other substrates rather stringent conditions are often employed. Chlorine and fluorine-based gas etchants are frequently used and extensive localized heating often occurs during the etching sequence. As a consequence, the patterned photoresist coating on the substrate can experience shrinkage or other degradation. Such degradation can reduce resolution of the feature etched into the substrate and even render the substrate unusable for its intended purpose.
Problems also can arise with use of positive-acting chemically amplified resists. These resists utilize a deprotecting or cleavage reaction of a resin binder component to provide solubility differentials between exposed and unexposed areas. That deprotection reaction can result in cleavage of a substantial mass of the resist which in turn can cause significant shrinkage of the resist coating layer areas exposed to activating radiation. Such shrinkage can compromise resolution of the image patterned in the photoresist coating layer. See, for example, U.S. Pat. Nos. 4,968,581; 4,883,740; 4,810,613 and 4,491,628 for a discussion of resist resin binders that contain acid labile groups that undergo deprotection or cleavage reactions in the presence of photogenerated acid.
Another persistent shortcoming of current resists is poor resolution of "isolated" resist lines or other features, particularly when using a positive resist. A developed resist line or other feature is generally considered "isolated" if it is spaced from the closest adjacent resist feature a distance equal to two or more times the line width. Thus, e.g., if a line is printed at a 0.25 .mu.m width, that line would be considered isolated (rather than dense) if the next adjacent resist feature was spaced at least about 0.50 microns from the line. Common resolution problems with isolated lines include rounded tops and undercutting.
It thus would be desirable to have new photoresist compositions, particularly resist compositions that exhibit enhanced stability or resistance to etching environments. It would be further desirable to have new chemically-amplified positive photoresist compositions that did not exhibit significant shrinkage during exposure and other processing steps. It also would be desirable to have new positive photoresist compositions that form highly resolved isolated lines.